1. Field of the Invention
The present invention relates to acoustic wave devices using acoustic waves, such as surface acoustic waves or boundary acoustic waves. More particularly, the present invention relates to an acoustic wave device including a temperature characteristic-improving layer arranged to adjust the temperature coefficient of frequency of the device that is disposed on a piezoelectric substrate.
2. Description of the Related Art
Surface acoustic wave devices are widely used as band filters for mobile phones. Japanese Unexamined Patent Application Publication No. 2001-44787 discloses an example of such a surface acoustic wave device.
FIG. 8 is a schematic front sectional view of the surface acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 2001-44787. The surface acoustic wave device 101 includes a piezoelectric substrate 102 and interdigital electrodes (IDT electrodes) 103 disposed thereon. An insulating protective layer 104 having a coefficient of linear expansion that is less than that of the piezoelectric substrate 102 extends over the interdigital electrodes 103. The insulating protective layer 104 is covered with a frequency-adjusting layer 105.
The insulating protective layer 104 reduces variations in frequency characteristics due to temperatures, that is, the insulating protective layer 104 is used as a temperature characteristic-improving layer. The insulating protective layer 104 is made of SiO2.
The frequency-adjusting layer 105 is made of SixNy having a wave velocity (a velocity of transverse wave of about 6000 m/s) greater than that of SiO2, wherein x and y depend on the composition thereof. Frequencies such as the center frequency and resonant frequency of the device are controlled by adjusting the thickness of the frequency-adjusting layer 105.
In the surface acoustic wave device 101 disclosed in Japanese Unexamined Patent Application Publication No. 2001-44787, the insulating protective layer 104 is used to improve the frequency-temperature characteristic and the frequency-adjusting layer 105 is used to control the frequencies as described above. With Si3N4 layers, each functioning as the frequency-adjusting layer 105, having a wave velocity (about 5850 to about 6010 m/s) and a thermal expansion coefficient (about 3.4×10−6/° C.) that are greater than those of SiO2 (a thermal expansion coefficient of about 0.55×10−6/° C.) are formed, there is a problem in that advantages due to the presence of SiO2 layers are deteriorated because the absolute value of each temperature coefficient of frequency (TCF), which indicates the frequency-temperature characteristic, particularly a variation in frequency due to temperature, is increased as shown in FIG. 9. With reference to FIG. 9, λ along the horizontal axis represents the wavelength of an acoustic wave, such as a surface acoustic wave or a boundary acoustic wave.